AbstractThe dengue virus consists of four antigenically related but distinct viruses, termed Dengue virus 1–4 (DEN 1–4). We have established that the dengue virus loses infectivity over time in solution in an exponentially declining manner. The four strains examined (one from each serotype) have half‐lives that range from 2.5 to 7.5 hr in defined medium. The half‐life is temperature and pH‐dependent and is affected by the nature of the host cell in which it is produced, but is not dependent upon the presence of either Mg2+ ions or chelating agents. Electron microscopy (EM) of solutions of the dengue virus show almost complete virus aggregation after 24 hr at room temperature, while RT‐PCR shows an intact RNA genome. These results show that the solution environment of the dengue virus is an important determinant of dengue virus infectivity. J. Med. Virol. 71:532–539, 2003. © 2003 Wiley‐Liss, Inc.